a Light-duty truck categories LDT1-LDT4 were not created
until 1994. From 1968 to 1978 all trucks with a GVWR up to 6,000 pounds were
classified as light-duty trucks and were required to meet the same standards.
As of 1979, the maximum weight was raised to 8,500 pounds GVWR. During 1988-93,
light-duty trucks were divided into two subcategories that coincide with the
current LDT1-LDT4 categories. The standards for LDT1, LDT3, and LDT4 are shown
in tables 4-30a, 4-40c, and 4-30d.

b The test procedure for measuring exhaust emissions
has changed several times over the course of vehicle emissions regulation. The
7-mode procedure was used through model year 1971 and was replaced by the CVS-72
procedure beginning in model year 1972. The CVS-75 procedure became the test
procedure as of model year 1975. While it may appear that the total HC and CO
standards were relaxed in 1972-74, these standards were actually more stringent
due to the more stringent nature of the CVS-72 test procedure. Additional standards
for CO and composite standards for NMHC and NOx tested over the new Supplemental
Federal Test Procedure will be phased-in beginning with model year 2000. These
standards are not shown in this table.

c Emissions standards had to be met for a useful life
of 5 years/50,000 miles through model year 1983, and a full useful life of 11
years/120,000 miles was defined for 1985-93 (several useful life options were
available for 1984). Beginning in model year 1994, emissions standards were
established for an intermediate useful life of 5 years/50,000 miles as well
as a full useful life (full useful life standards are shown in parentheses).
HC standards, however, were established only for full useful life. Tier 1 exhaust
standards, except particulates standards, were phased-in during 1994-96 at a
rate of 40%, 80%, and 100%, respectively. Particulates standards were phased-in
at a rate of 40%, 80%, and 100% during 1995-97.

d The cold CO emissions standard is measured at 20
degrees F (rather than 75 degrees F) and is applicable for a 5-year/50,000-mile
useful life.

f Manufacturers can opt to certify vehicles for a full
useful life of 15 years/150,000 miles and either have (1) intermediate useful
life standards waived or (2) receive additional NOx credits.

g The "Prior to controls" reports emissions
estimates of a typical newly manufactured car in the years before exhaust emissions
certification standards were implemented.

h No estimate available.

iIn 1968-69, exhaust emissions standards
were issued in parts per million rather than grams per mile and are, therefore,
incompatible with this table.

jNo
standard has been set.

k The term "tier" refers to a level of standards
for specific years. Interim 2 refers to an intermediate level of standards that
move manufacturers toward compliance with Tier 2 standards. Interim Tier 2 and
Tier 2 standards are established as "bins." Each bin is a set of standards
for NOx, CO, NMOG, formaldehyde, and particulates (HC and NMHC standards are
dropped for Tier 2 and Interim Tier 2). Manufacturers may certify any given
vehicle family to any of the bins available for that vehicle class as long as
the resulting sales-weighted corporate average NOx standard is met for the full
useful life. The Tier 2 corporate average NOx standard is 0.07 grams/mile. Interim
corporate-based average NOx standards are based on vehicle type. The interim
corporate sales-weighted average for LDT2 vehicles is 0.3 grams/mile. Tier 2
standards will be phased in at a rate of 25% in 2004, 50% in 2005, 75% in 2006,
and 100% in 2007. During this period all LDT2 vehicles not meeting the Tier
2 standards must meet Interim Tier 2 standards.